The present invention relates to fiber-optic communications networks, and more particularly, to grating-based dispersion compensators for optical network equipment.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify optical signals. For example, optical amplifiers may be used to amplify optical data signals that have been subject to attenuation over fiber-optic paths. A typical amplifier may include erbium-doped fiber coils that are pumped with diode lasers. Raman amplifiers have also been investigated. Discrete Raman amplifiers may use coils of dispersion-compensating fiber to provide Raman gain. Distributed Raman amplifiers provide gain in the transmission fiber spans that are used to carry optical data signals between network nodes.
Dispersion compensation elements are used in fiber-optic links to compensate for the effects of chromatic dispersion. One popular type of dispersion compensation element is based on dispersion-compensating fiber. A coil of about 10-15 km of dispersion-compensating fiber may be installed as a module in the midstage of an optical amplifier to compensate for the accumulated chromatic dispersion on a preceding span of optical transmission fiber in a communications link. Dispersion compensation modules and optical amplifiers that include dispersion-compensating fiber may, however, be bulky.
It is an object of the present invention to provide improved dispersion compensation elements for use in optical network equipment such as optical amplifiers.
It is another object of the present invention to provide grating-based dispersion compensation elements.
These and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers and other optical network equipment in which dispersion compensation elements based on fiber gratings may be used. The dispersion compensation elements may be provided in the form of dispersion compensation modules that may be installed, for example, at midstage access ports in optical amplifiers.
The fiber gratings may be based on a chirped superstructure configuration that allows the dispersion compensation devices to cover a wide signal bandwidth with good performance. The superstructure grating configuration includes a number of grating portions or samples that are organized to form a larger grating pattern.
In order for each grating to provide dispersion compensation that covers a sufficiently wide bandwidth and a sufficiently wide channel size, the gratings may be chirped in two respects. First, a relatively small amount of chirp may be impressed linearly along the length of the entire grating across all of the samples in the superstructure. This chirp contribution ensures that wavelength-division-multiplexing communications channels of normal bandwidths (e.g., 25-100 GHz) may be adequately covered. Second, a relatively large amount of chirp may be impressed across each grating sample. This chirp contribution helps to ensure that the total bandwidth covered by the grating is sufficiently wide (e.g., 30-40 nm). The total bandwidth may cover the entire C-band or L-band or other suitable signal band in an optical amplifier and communications link.
The gratings may be fabricated using by using an excimer laser or other suitable ultraviolet (UV) light source to impress grating patterns on a photosensitive fiber through a mask. The mask may include the relatively small chirp contribution. A lens arrangement may be used that relies on cylindrical lens aberrations to help create the desired relatively large chirp contribution.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.